Efficient synthesis and properties of isomeric photochromic diarylethenes having a pyrrole unit

Article abstract of DOI:10.1021/ol070622q

Two new isomeric photochromic diarylethenes with a pyrrole unit have been developed. Their properties, including photochromic behavior, crystal structures, and fluorescent properties, have been investigated. The two isomeric compounds show distinctly different photochromism, both in solution and in the single-crystalline phase: one turns red upon photocyclization whereas the other turns blue, which may be attributed to the different substituent effects.

Full text of DOI:10.1021/ol070622q

ORGANIC
LETTERS
2007
Vol. 9, No. 11
2139-2142
Efficient Synthesis and Properties of
Isomeric Photochromic Diarylethenes
Having a Pyrrole Unit
Shouzhi Pu,* Gang Liu, Liang Shen, and Jingkun Xu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal
UniVersity, Nanchang 330013, People’s Republic of China
pushouzhi@tsinghua.org.cn
Received March 14, 2007
ABSTRACT
Two new isomeric photochromic diarylethenes with a pyrrole unit have been developed. Their properties, including photochromic behavior,
crystal structures, and fluorescent properties, have been investigated. The two isomeric compounds show distinctly different photochromism,
both in solution and in the single-crystalline phase: one turns red upon photocyclization whereas the other turns blue, which may be attributed
to the different substituent effects.
Photochromic diarylethene derivatives have attracted much
attention as highly promising candidates for optoelectronic
devices such as optical memories and photoswitches, mainly
due to the excellent thermal stability of the respective
isomers, notable fatigue resistance, and high reactivity in the
solid state.1 Recently, the design and synthesis of novel
diarylethene systems with different aryl moieties has become
an active area of research. Among the diarylethenes hitherto
reported, most of the heteroaryl moieties have been thiophene
or benzothiophene rings,2 with just a few reports concerning
other heteroaryl moieties, such as furan,3 thiazole,4 indole,5
benzofuran,6 indene,7 pyrazole,8 etc. The majority of the
research work reported to date has been devoted to the
development of these molecules and investigative studies of
their fundamental properties, and the results obtained have
contributed to a broad understanding of the photochromism
of diarylethenes.9 At the same time, the results have also
demonstrated that the photochromic performance of each type
of diarylethene is strongly dependent on the nature of the
aryl groups. For instance, diarylethenes with thiophene or
benzothiophene moieties exhibit excellent thermal stability
and outstanding fatigue resistance,1 whereas diarylethenes
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Chem. Soc. ReV. 2004, 33, 85. (c) Kawata, S.; Kawata, Y. Chem. ReV. 2000,
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Org. Lett. 2005, 7, 3777. (e) Zhou, Z. G.; Xiao, S. Z.; Xu, J.; Liu, Z. Q.;
Shi, M.; Li, F. Y.; Yi, T.; Huang, C. H. Org. Lett. 2006, 8, 3911. (f) Jeong,
Y. C.; Yang, S. I.; Kim, E.; Ahn, K. H. Tetrahedron 2006, 62, 5855. (g)
Kim, E.; Kim, M.; Kim, K. Tetrahedron 2006, 62, 6814.
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1998, 54, 6627. (b) Takami, S.; Irie, M. Tetrahedron 2004, 60, 6155.
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K.; Soong, C. F.; Irie, M. J. Org. Chem. 2001, 66, 5419.
(6) Yamaguchi, T.; Irie, M. J. Org. Chem. 2005, 70, 10323.
(7) Yamaguchi, T.; Irie, M. Tetrahedron Lett. 2006, 47, 1267.
(8) (a) Pu, S. Z.; Yang, T. S.; Xu, J. K.; Chen, B. Tetrahedron Lett.
2006, 47, 6473. (b) Yang, T. S.; Pu, S. Z.; Chen, B.; Xu, J. K. Can. J.
Chem. 2007, 85, 12.
(9) Lee, J. K.-W.; Ko, C.-C.; Wong, K. M.-C.; Zhu, N. Y.; Yam, V.
W.-W. Organometallics 2007, 26, 12.
10.1021/ol070622q CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/02/2007

with a pyrazole unit have a relatively high cycloreversion
quantum yield.8 Pyrrole is an attractive aryl unit because of
its biological characteristics, and its structure is similar to
those of thiophene and furan. As far as we are aware, there
has been only one example of a photochromic pyrrole
derivative, as reported by Uchida et al.10 They synthesized
a symmetrical diarylethene bearing two pyrrole rings, but
indicated that it was thermally unstable and reverted to the
open-ring form even at room temperature in the dark. To
the best of our knowledge, photochromic hybrid diarylethene
derivatives bearing both pyrrole and thiophene moieties have
not hitherto been reported.
Scheme 2. Synthetic Route to 1 and 2
Generally, all diarylethene compounds reported to date
have seemingly been synthesized separately, i.e., only one
target product was obtained following a series of synthetic
operations. The cognate synthesis of two or more isomeric
diarylethenes by a parallel series of synthetic operations has
never been reported. In this Letter, we describe the parallel
preparation of two new hybrid isomeric diarylethenes bearing
a pyrrole unit, namely 1-[2-methyl-5-(4-methoxyphenyl)-3-
thienyl]-2-(2-cyano-1,5-dimethyl-3-pyrryl)hexafluorocyclo-
pentene (1a) and 1-[2-methyl-5-(4-methoxyphenyl)-3-thienyl]-
2-(2-cyano-1,5-dimethyl-4-pyrryl)hexafluorocyclopentene (2a)
(Scheme 1). Interestingly, these two isomeric diarylethenes
hexane are shown in Figure 1. Upon irradiation with 297
nm light, the colorless solution of 1a turned red due to the
appearance of a new visible absorption band centered at 537
nm attributable to the closed-ring isomer 1b. The red solution
reverted to colorless upon irradiation with visible light (λ >
450 nm), indicating that 1b returned to the initial state 1a,
and a clear isosbestic point was observed at 314 nm.
Similarly, upon irradiation with 297 nm light, the colorless
solution of 2a turned blue and an absorption maximum
developed at 592 nm as the closed-ring isomer 2b was
generated. The blue color disappeared upon irradiation with
visible light (λ > 450 nm), and an isosbestic point was
observed at 317 nm. The cyclization and cycloreversion
quantum yields of 1 were determined to be 0.53 and 0.44,
respectively. The corresponding cyclization and cyclorever-
sion quantum yields of 2 were measured as 0.55 and 0.10,
respectively. It is notable that the cycloreversion quantum
Scheme 1. Mechanistic Rationale of the Photochromic
Property of 1 and 2
1 and 2 show distinctly different photochromic properties
both in solution and in the single-crystalline phase.
The synthetic route to the photochromic diarylethenes 1a
and 2a is illustrated in Scheme 2. First, compounds 4 and 5
were obtained in a combined yield of 78% by brominating
compound 1 in acetic acid at room temperature. Lithiation
of the mixture of 4 and 5 followed by the addition of excess
octafluorocyclopentene simultaneously generated compounds
6 and 7, which could be further treated with the anion gen-
erated from 3-bromo-2-methyl-5-(4-methoxyphenyl)thiophene
(8)11 to yield isomeric diarylethenes 1a and 2a. Their
structures were confirmed by NMR, MS, and elemental
analysis, IR spectroscopy, and X-ray single-crystal diffraction
analysis (Supporting Information).
The changes in the absorption spectra of diarylethenes 1
and 2 induced by photoirradiation at room temperature in
(10) Uchida, K.; Matsuoka, T.; Sayo, K.; Iwamoto, M.; Hayashi, S.; Irie,
M. Chem. Lett. 1999, 835.
(11) (a) Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem.
1995, 60, 8305. (b) Sun, F.; Zhang, F. S.; Guo, H. B.; Zhou, X. H.; Wang,
R. J.; Zhao, F. Q. Tetrahedron 2003, 59, 7615.
Figure 1. Changes in the absorption spectra of diarylethenes 1
and 2 in hexane solution at room temperature (2.0 × 10-5 M).
2140
Org. Lett., Vol. 9, No. 11, 2007

yield of 2 is lower than that of 1. These facts indicate that,
in solution, the two isomeric compounds show remarkable
differences in their photochromic behavior. The photochro-
mic properties of diarylethenes 1 and 2 are summarized in
Table 1. From these data, it can easily be seen that the
group (OCH3) through the ethylene moiety. As a result, 2b
becomes a push-pull conjugated system (D-π-A), with
the strongly electron-withdrawing cyano group and the elec-
tron-donating methoxy group as the acceptor and donor, re-
spectively. Therefore, 2b exhibits a more marked intramo-
lecular charge transition (ICT) than 1b, which is consistent
with the red-shifted absorption maximum and the strong
molar absorption coefficient of 2b in comparison with those
of 1b.
The thermal stabilities of the open-ring and closed-ring
isomers of 1 and 2 were examined by storing solutions in
hexane at room temperature in the dark and then exposing
them to air for more than 2 months. In all cases, no changes
in their UV/vis spectra were observed and, indeed, no
decomposition was detected when these compounds were
exposed to air for more than a year. The fatigue resistances
of 1 and 2 were examined in air at room temperature.
Solutions of 1 and 2 in hexane were alternately irradiated
with light of wavelength 297 nm for 200 s and with visible
light (λ > 450 nm) for 60 s. The fatigue-resistant charac-
teristics of 1 and 2 in solution (see the Supporting Informa-
tion) indicated that 90% of each of 1b and 2b was destroyed
after 12 repeat cycles. This may be ascribed to degradation
resulting from the formation of an epoxide.13 However, the
fatigue resistances of 1 and 2 in the solid state are very
strong. After 500 repeat cycles, the two isomeric compounds
still showed good photochromism with only ∼20% degrada-
tion of each of 1b and 2b, both in PMMA film and in the
single-crystalline phase (see the Supporting Information).
This remarkable improvement may result from effectively
suppressing the oxygen diffusion.1a
Table 1. Photochromic Parameters of Compounds 1 and 2 in
Hexane at 2.0 × 10-5
M
φc
λmax/nma
λmax/nmb
(ꢀ/L mol-1 cm-1
compd
(ꢀ/mol-1 cm-1
)
)
φa-b
φb-a
1
2
288 (2.3 × 104)
537 (3.9 × 103)
0.53
0.44
0.55
0.10
296 (2.8 × 104)
592 (9.0 × 103)
a Absorption maxima of open-ring forms. b Absorption maxima of closed-
ring forms. c Quantum yields of cyclization (φa-b) and cycloreversion (φb-a),
respectively.
photochromic features (including the absorption maxima,
molar absorption coefficients, and quantum yields) of diaryl-
ethenes 1 and 2 are distinctly different.
To further understand the differences between isomers 1
and 2, we compared the photophysical properties of the
closed-ring isomer 1b with those of 2b. Compared to those
of 1b, the absorption maximum of 2b is red-shifted by 55
nm and the molar absorption coefficient of 2b is threefold
larger. The results indicate that the positions of the methyl
(CH3) and cyano (CN) substituents have a significant effect
on the photochromic properties of the two isomeric diaryl-
ethene compounds. Although substituent effects on the
photochromic behavior of diarylethene derivatives have been
reported previously,1a,8,10,12 no such substituent effect has
hitherto been reported for isomers of diarylethene derivatives.
Figure 2 indicates the extent of the conjugation in the
closed-ring isomers 1b and 2b. In the case of 1b, the cyano
Single crystals of 1a and 2a were obtained by recrystal-
lization from chloroform. Their ORTEP drawings are shown
in Figure 3, which indicate that both of them are packed in
Figure 2. Sketches of the extent of conjugation in the closed-ring
systems 1b and 2b.
group was involved in the cyclization reaction, and the
weakly electron-donating methyl group participates in the
conjugation system. In the case of 2b, the methyl group was
involved in the cyclization reaction, and the strongly electron-
withdrawing cyano group is conjugated with the methoxy
Figure 3. ORTEP drawings of 1a and 2a (ellipsoids are drawn at
the 35% level).16
(12) (a) Morimitsu, K.; Shibata, K.; Irie, M. J. Org. Chem. 2002, 67,
4574. (b) Uchida, K.; Matsuoka, T.; Kobatake, S.; Yamaguchi, T.; Irie, M.
Tetrahedron 2001, 57, 4559. (c) Takami, S.; Irie, M. Tetrahedron 2004,
60, 6155. (d) Pu, S. Z.; Yang, T. S.; Xu, J. K.; Shen, L.; Li. G. Z.; Xiao,
Q.; Chen, B. Tetrahedron 2005, 61, 6623.
anti-parallel conformations14 in the crystalline phase. The
distances between the two reactive carbons of 1a (C5‚‚‚C9)
Org. Lett., Vol. 9, No. 11, 2007
2141

and 2a (C5‚‚‚C14) are 3.69 and 3.54 Å, respectively.
Generally, such a molecule can undergo photocyclization if
it is fixed in an anti-parallel mode and the distance between
the reacting carbon atoms of the aryl rings is less than 4.2
Å.1f,15 Therefore, 1a and 2a exhibit photochromism in the
crystalline phase. This was verified by the fact that photo-
irradiation of single crystals of 1a and 2a resulted in observ-
able color changes (Figure 4). Upon irradiation with 297 nm
of 1a and 2a were determined as 0.019 and 0.016, respec-
tively, using anthracene (0.27 in acetonitrile) as a reference.
Thus, it is clear that the position of the substituent on the
pyrryl group has a significant effect on the emission intensity,
but does not noticeably affect the position of the emission
peak. In addition, 1 and 2 exhibited changes in their
fluorescences during the process of photoisomerization, as
has been observed for most of the reported diarylethenes.17
The emission intensities of 1a and 2a were decreased by
photocyclization upon irradiation with 297 nm light. The
emission intensities of 1 and 2 in a photostationary state were
quenched to ca. 65% and 48%, respectively. The compara-
tively low cyclization conversions and the existence of
parallel conformations of 1a and 2a may be the main cause
for the moderate change in fluorescence induced by pho-
toirradiation. Compound 2 shows a more marked change in
fluorescence upon photocyclization than compound 1 because
of its higher absorption efficiency.
In summary, two new isomeric diarylethenes with a pyrrole
unit have been synthesized concomitantly by a parallel series
of reaction steps and their properties have been investigated.
This new photochromic system showed good thermal stabil-
ity and strong fatigue resistance in the solid state. Impor-
tantly, the two isomeric compounds showed remarkable
differences in their photochromic and fluorescent properties,
both in solution and in the single-crystalline phase. The
results of this study may be useful for the design and
synthesis of efficient photoactive diarylethene derivatives
with tunable properties.
Figure 4. Photographs demonstrating the photochromic processes
of diarylethenes 1 and 2 in the crystalline phase.
light, colorless crystals of 1a and 2a became red and blue,
respectively. The colored crystals returned to colorless upon
irradiation with visible light of the appropriate wavelength.
The photographs in Figure 4 clearly demonstrate that the
photochromic processes of diarylethenes 1 and 2 occur in
the crystalline phase.
Furthermore, the fluorescence spectra of 1a and 2a in
hexane at room temperature are illustrated in Figure 5. Both
compounds show good fluorescence at their respective
excitation wavelengths. The fluorescent emissions of 1a and
2a were observed at 423 and 421 nm upon excitation at 335
and 320 nm, respectively. The fluorescence quantum yields
Acknowledgment. The authors thank the NSFC of China
(20564001, 50503009, and 50663001) and the NSFJC
(050017) for financial support.
Supporting Information Available: Synthetic and ex-
perimental details and a CIF file for 1a and 2a. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL070622Q
(13) Higashiguchi, K.; Matsuda, K.; Yamada, T.; Kawai, T.; Irie, M.
Chem. Lett. 2000, 1358.
(14) (a) Irie, M.; Miyatake, O.; Uchida, K. J. Am. Chem. Soc. 1992,
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(16) Crystal data for 1a: C24H18.67F6N2O1.33S: M ) 502.47, monoclinic
(P21/c), a ) 14.856(18) Å, b ) 22.866(3) Å, c ) 21.756(3) Å, R ) 90°,
â ) 102.71(2)°, γ ) 90°, V ) 7209.7(15) Å3, Z ) 12, µ ) 0.202 mm-1
,
R1 [I > 2σ(I)] ) 0.0844, wR2 [I > 2σ(I)] ) 0.2141, R1 (all data) ) 0.2253,
wR2 (all data) ) 0.3204, GOF ) 0.999. The CCDC number of 1a is 639785.
Crystal data for 2a: C24H18F6N2OS: M ) 496.46, monoclinic (P21/c), a )
23.120(3) Å, b ) 8.947(11) Å, c ) 11.442(13) Å, R ) 90°, â ) 101.84-
(10)°, γ ) 90°, V ) 2316.6(5) Å3, Z ) 4, µ ) 0.207 mm-1, R1 [I > 2σ(I)]
) 0.0537, wR2 [I > 2σ(I)] ) 0.1364, R1 (all data) ) 0.0715, wR2 (all
data) ) 0.1524, GOF ) 1.038. The CCDC number of 2a is 639786.
(17) (a) Frigoli, M.; Mehl, G. H. Chem. Eur. J. 2004, 10, 5243. (b) Pu,
S. Z.; Yang, T. S.; Li, G. Z.; Xu, J. K.; Chen, B. Tetrahedron Lett. 2006,
47, 3167. (c) Xiao, S. Z.; Yi, T.; Zhou, Y. F.; Zhao, Q.; Li, F. Y.; Huang,
C. H. Tetrahedron 2006, 62, 10072.
Figure 5. Fluorescence spectra of 1a and 2a in hexane (1.0 ×
10-4 M) at room temperature, excited at 335 and 320 nm,
respectively.
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Org. Lett., Vol. 9, No. 11, 2007